1. Field of the Invention
The present invention relates to a multimedia broadcast and multicast services, more particularly to a channel switching methods of Multimedia Broadcast and Multicast P-t-P and P-t-M channel.
2. Description of the Prior Art
Multimedia Broadcast and Multicast Service (MBMS) is a new Point to Multipoint (P-t-M) service in 3rd generation (3G) mobile communication. The type of channel used by MBMS can be a Point to Point (P-t-P) channel or a P-t-M channel according to the number of service users. In current P-t-P service mechanisms, when a user moves to a new cell, a destination cell, a specific channel is used for handovers from the source cell to the destination cell. Existing channel handovers are all based on P-t-P channel and the channel characteristics don't change. In MBMS service, the changing cell may result in changes of channel characteristics.
To better explain the relevant principles, the MBMS system structure is illustrated in FIG. 9.
MBMS network structure adds new network elements based on the core network of General Packet Radio Service (GPRS). Referring to FIG. 9, a broadcast and multicast service center (BM-SC) 01 is the service control center of the MBMS system. A Gateway GPRS Supporting Node (GGSN) 02 and Service GPRS Supporting Node (SGSN) 03 consist of the transmission network of MBMS service and provide a route for data transfer. A Home Location Register (HLR) 06 stores the user-related data and can provide services like user authentication. A UMTS Terrestrial Radio Access Network (UTRAN) 04 provides radio resources for MBMS service in air interface. Uu 07 denotes the radio interface between terminal and radio access network. User Equipment (UE) 05 is the terminal device receiving data. Radio resources used by the MBMS service are not dedicated for one user, but are shared by all users of the service.
In a conventional system, it is up to the Serving Radio Network Controller (SRNC) to decide whether to do the SRNC relocation when the user in the system performs a handover between cells in different RNCs but in the same SGSN. If SRNC relocation is not performed, data and signalling stream are transferred to the Drift Radio Network Controller (DRNC) via Iur interface, and then transferred to the UE from the DRNC. In a handover procedure, different processes are performed according to the different radio connection status of the user.
In Idle mode, CELL_FACH (Cell_Forward Access Channel), CELL_PCH (Cell_Paging Channel) and URA_PCH (UTRAN Registration Area_Paging Channel) statuses, when the UE moves from one cell in the SRNC to a cell in another RNC, this is a drift that is handled by the DRNC. The UE will send a “cell update” message to the DRNC, and the DRNC transfers the message to the SRNC via an Iur interface. The SRNC will communicate with the DRNC to obtain common resource information of the cell in the DRNC to notify the UE. FIG. 7 illustrates the cell update process, which is as follows:    Step 601: After the UE re-selects a cell, it sends a cell update message to the DRNC via common control channel (CCCH) using a radio resource control (RRC) protocol.    Step 602: The DRNC allocates a D-RNTI (DRNC-Radio Network Temporary Identifier) to the UE and then sends an uplink signalling transfer indication message to the SRNC using a radio network subsystem application part (RNSAP).    Step 603: The SRNC decides not to do SRNC relocation and sends a common transport channel resource initialization request message to the DRNC.    Step 604: The DRNC informs the SRNC of the information on a common channel through the message of common transport channel resource initialization response.    Step 605: The SRNC sets up a data bearer on an Iur interface using an access link control application part (ALCAP).    Step 606: The SRNC sends a cell update confirmation message to the UE to inform the UE common channel of information on the new cell and new UE identifier via a dedicated control channel (DCCH) using RRC protocol.    Step 607: The UE responds to the SRNC to complete the cell update process via the DCCH.    Step 608: The SRNC deletes the user's resources on the original cell using RNSAP by sending a common transport channel resources message.
In CELL_DCH (Cell_Dedicated Channel) status, the UE moves to a cell in another RNC. The SRNC decides to handover to another cell according to the measurement reported by the UE and notifies the DRNC of dedicated radio link configurations. After the DRNC completes configuring, the SRNC is informed to notify the UE of the radio link situation after UE handover. FIG. 8 illustrates the cell handover process under this status that occurs as follows:    Step 701: The SRNC decides to set up a new dedicated radio link (RL) for the UE in the new cell of the DRNC. SRNC sends a radio link setup request message to DRNC using RNSAP and informs the DRNC of dedicated channel information.    Steps 702-703: The DRNC sets up a new radio dedicated link with a Node B of the DRNC using a Node B application part (NBAP).    Step 704: The DRNC sends a radio link addition response message to SRNC.    Step 705: The SRNC sets up Iur interface and Iub interface data bearer using ALCAP.    Steps 706-707: Synchronize data frame on downlink and uplink using DCH frame pulse (FP).    Steps 708-709: SRNC notifies the UE to add a new dedicated link via the DCCH by sending an active set update message using the RRC. The UE sends active set update complete message in response to the active set update message.
In conventional systems, the user's signaling and data are all sent to the DRNC via the SRNC, and then the DRNC sends them to the user. MBMS is a new P-t-M service that can use a P-t-P or P-t-M channel for data transfer. The RNC determines channel type according to the number of users applying for the same kind of MBMS service in a cell. When the number of users using the same kind of MBMS service is small, the P-t-P channel is used; when the user number for this service exceeds a certain threshold, the P-t-M channel is used.
Therefore, when the user moves from an SRNC cell to a DRNC cell during cell reselection, the SRNC does not relocate. The DRNC determines the type of MBMS channel, which may result in different MBMS channel types being used before and after handover. So it is necessary to reconfigure MBMS channels used by the user from P-t-P channel to P-t-M channel. Conventional handover technology cannot handle this situation.
After the UE completes the above cell handover process, if the number of MBMS service users change due to the cell users moving in and out of the cell, or other users joining in and leaving the same MBMS service, and the threshold of the number of users is exceeded, the MBMS channel type will change. The current state of the art cannot notify the SRNC of MBMS channel changes and does not allow the SRNC to notify all users in the cell to reconfigure the MBMS channel.
The RNC is responsible for counting the number of users applying for MBMS service. If a handover is made from a cell in the SRNC to a cell in the DRNC during cell handover, SRNC re-positioning is not performed. This presents another problem to be solved since the SRNC needs to notify the DRNC of the MBMS service type applied for by the user to make the DRNC re-count the number of MBMS service users in the cell. Since MBMS is a new service, the conventional handover mechanism is not suitable and sufficient for the MBMS service.